In electrical equipment, and in particular in metal-clad equipment, including at least one set of bus bars and a circuit breaker, and fitted in conventional manner with a protective system and a computer, it is known to use a sensor to detect the appearance of an arc between a conductor and the metal case (or between conductors in a three-phase installation) where such an arc is said to be "internal". The sensor may be a pressure detector or a light detector. Such devices are described in patents EP-0 197 778 and EP-0 313 422.
Nevertheless, while the contacts of the circuit breaker are interrupting a load current or a fault current, the arc which is struck between its contacts, referred to as the "interruption" arc, has characteristics that are similar to those of an internal arc. The increase in light or pressure while interrupting a high short circuit current is close to that created during the appearance of a quickly eliminated internal arc with a low short circuit current.
This makes distinguishing between an internal arc and an interruption arc uncertain when it is performed merely by measuring an increase in pressure or of light.